The present disclosure relates to exhaust-gas purification catalytic systems.
Hybrid automobiles using both engines and electric motors as drive sources can reduce emission which is an environmental load. In addition, it is also important to further efficiently purify exhaust gas from engines. Automobiles whose engines operate near the stoichiometric air-fuel ratio conventionally use three-way catalysts capable of purifying hydrocarbon (HC), CO, and nitrogen oxide (NOx) at the same time. As catalytic metals, Pt, Pd, and Rh, for example, have been adopted.
In such a three-way catalyst, purification performance for HC, CO, and NOx described above degrades when the air-fuel ratio of an exhaust gas from an engine comes to be in a lean or rich condition. To prevent this degradation, the three-way catalyst includes an oxygen storage/release material which stores oxygen when the ambience of the catalyst is in a lean condition, and releases oxygen when the ambience of the catalyst is in a rich condition. That is, the storage and release of oxygen can control the air-fuel ratio of the ambience of the catalyst toward stoichiometry, thereby preventing degradation of performance in purifying an exhaust gas. In addition, since this Ce-containing oxide alone is limited in storage/release amount of oxygen, a component promoting oxygen storage is loaded on the surface of the Ce-containing oxide. This component is, for example, Pt, Pd, or Rh.
Specifically, Pt, Pd, and Rh function not only as catalytic metals promoting oxidation of HC and CO and reduction of NOx, but also as components controlling the air-fuel ratio of the ambience of the three-way catalyst together with an oxygen storage/release material so as to allow the three-way catalyst to easily exhibit purification performance. However, since Pt, Pd, and Rh are scarce resources, it is required for a catalyst to exhibit a high exhaust-gas purification performance with reduced amount of Pt, Pd, and Rh.
In a known exhaust-gas purification catalytic system, three-way catalysts are disposed at an upstream side and a downstream side in an exhaust-gas flow in an exhaust passage of an engine. For example, in such a system, a so-called close-coupled catalyst coupled to a downstream end of an exhaust manifold and a so-called underfloor catalyst placed under the automobile floor at a downstream side in an exhaust-gas flow are used to purify an exhaust gas. In this catalytic system, the upstream close-coupled catalyst rapidly comes to have a temperature at which this close-coupled catalyst is active by heat of the exhaust gas, and therefore, is effective for purification of an exhaust gas discharged at, for example, a start-up of the engine and having a relatively low temperature. On the other hand, the downstream underfloor catalyst also has a temperature at which this underfloor catalyst is active around a time when a relatively large amount of a high-temperature exhaust gas is discharged from the engine. Accordingly, cooperation of the close-coupled catalyst and the underfloor catalyst can efficiently purify an exhaust gas.
A catalytic system constituted by upstream and downstream catalysts as described above is described in, for example, Japanese Patent Publication No. 2006-291918. Specifically, in this publication, a Rh-doped CeZr-based composite oxide is adopted as an oxygen storage/release material for an upstream catalyst, thereby achieving a structure in which the oxygen storage material for the upstream catalyst stores a larger amount of oxygen than that for the downstream catalyst under the same temperature. This structure is intended to reduce the size, or improve the layout, of the system while enhancing exhaust-gas purification performance and ensuring thermal resistance. This publication describes that enhancement of performance of the oxygen storage/release material for the upstream catalyst allows excellent exhaust-gas purification performance to be obtained even in a case where the air-fuel ratio of an exhaust gas varies in a wide range.
Japanese Patent Publication No. 2008-62156 describes that a material in which Rh is loaded on the surface of Rh-doped CeZrNd composite oxide is adopted as an oxygen storage/release material for a downstream catalyst, and an oxygen storage/release material capable of storing a smaller amount of oxygen than Rh-doped CeZrNd composite oxide is adopted for an upstream catalyst. This structure is intended to solve a problem in which Rh loaded on the surface of the Rh-doped CeZrNd composite oxide is oxidized by active oxygen released from this composite oxide with a variation of the air-fuel ratio to fail to return to a reduction state in which the catalyst activity is high. Specifically, Rh-loading Rh-doped CeZrNd composite oxide is adopted not for the upstream catalyst but for the downstream catalyst, and HC and CO flowing which have slipped through the upstream catalyst to flow toward a downstream side are used as reducing agents and Rh loaded on the surface of the Rh-doped CeZrNd composite oxide is maintained in a highly active state. In addition, activated alumina loading Pt is provided in the downstream catalyst such that HC described above is oxidized to be converted into, for example, CO having high reducing power, thereby activating (reducing) Rh.
The structure of the catalytic system in the above publication significantly enhances light-off characteristics (i.e., reducing the light-off temperature) for purification of HC, CO, and NOx and purification performance at high temperatures, thereby obtaining a high purification efficiency even in a case where the air-fuel ratio of an exhaust gas varies. However, in operation of automobiles, the air-fuel ratio of an exhaust gas might rapidly become a lean condition due to a fuel cut at deceleration in some cases. In such cases, purification of NOx degrades.